This invention relates to methods for compression molding lignocellulosic materials into articles, such as panels, boards, sheets, non-planar and multi-planar items, and the like, and more particularly, to such methods employing a lignocellulosic material having a high moisture content and a relatively low press temperature.
It is well known to manufacture so-called particle board or flakeboard by blending a suitable binder with wood flakes or other comminuted lignocellulosic materials, forming the resulting furnish into a mat and compressing the mat between heated platens to set the binder and bond the flakes together in a densified form. Conventional methods typically employ a press temperature of 300.degree. to 400.degree. F. and require the flakes to have a moisture content of about 2 to 8% prior to blending.
This requirement for low moisture content increases operating costs and adversely affects the quality of the final product. For instance, flakeboard panels made by conventional processes often are virtually oven dry, i.e., have a moisture content on the order of 1%, after completion of the press operation. Such panels are commonly used in an environment where the humidity is substantially higher. Consequently, the panels tend to absorb moisture and swell in thickness, particularly at the edges. If saturated with water and then dried, the panels do not return to the original thickness and remain permanently swollen.
Use of flakes or particles having a higher moisture content and a conventional press temperature can cause a rapid release of steam generated from the moisture in the mat, resulting in interior delaminations or "blows".
Representative prior methods for compression molding composite articles from lignocellulosic materials are disclosed in the following United States Patents:
______________________________________ Patents U.S. Pat. No. Issue Date ______________________________________ Barnes 4,751,131 June 14, 1988 Janiga 4,701,383 Oct. 20, 1987 Fuzesi, et al. 4,490,517 Dec. 25, 1984 Shaner, et al. 4,361,612 Nov. 30, 1982 Gaul, et al. 4,344,798 Aug. 17, 1982 Lund, et al. 4,241,133 Dec. 23, 1980 Diehr, et al. 3,870,665 Mar. 11, 1975 ______________________________________
PCT Publication No. 0 039 137 discloses another representative prior art process in which an encapsulated accelerator is admixed with an isocyanate binder prior to blending with the wood flakes to reduce the press time.
It has been proposed that the use of wood flakes having a moisture content of 15% or higher and lower press temperatures might alleviate the above-described shortcomings of many conventional processes. Palardy, et al., "Low Temperature and High Moisture Content Production Developments of Wood Composite Panels", Abstract of Presentation at Annual Meeting of Forest Products Research Society, Louisville, Ky., June, 1987, discloses the use of wood flakes having a moisture content of about 25%, a polyisocyanate binder and a press temperature of 210.degree. F. However, a press time of 9 minutes or more was required to produce 1/2 inch thick panels, even though a tertiary amine was combined with the binder prior to blending with the flakes in an attempt to accelerate curing. At this thickness, a press time more than about 300 seconds generally is not acceptable for commercial applications.